Flaws, Flaws, and more flaws. And staying consistent with those flaws, even when they are a detriment to the character. I try to practice what I preach, and have lost many a treasured character because of this.
Nothing bores me more than a flawless character, for instance:
"H'lo, Ahm a rough and tough waster who kicks ass, takes names, and accounts to no one... But I know everything about the known world, flora, fauna, magick, foreign lands, aristocracy, and every other subject. Btw when I talk to a templar, I lose my gritty speech patterns, and learn 24235424 new words."